通用收集&通配符在java中 [英] Generic collection & wildcard in java

问题描述

我遇到以下情况时遇到问题，请参阅以下内嵌注释：

  void exampleMethod（）{
 //直观地，我希望这意味着测试设置包含对象
 //子类AbstractGroup 
 Set< ;? extends AbstractGroup>测试; 
 
 //但编译器在这里抱怨，我不明白为什么？ 
 
 test.add（new AnyAbstractGroupSubGroup（））; 
 
 //我猜想在运行时这样的方法调用
 
 test = new HashSet< SubGroupA>（）
 
 //意味着只有subgroupA的对象可以添加到集合，但是
 //首先使用通配符是什么意思？ 
} 
  

解决方案

  //直观地，我希望这意味着测试设置包含对象
 //子类AbstractGroup 
 Set< ;? extends AbstractGroup>测试; 
  

不，这意味着它是一个特定的集合？它扩展了AbstractGroup。你和编译器都没有什么办法知道什么？

您可以将集合的值赋给AbstractGroup类型的变量，而不是相反。 p>

您需要这样：

  Set< ;? super AbstractGroup>测试; 
  

这个原则有时被称为 PECS ，并解释得很好在此答案。

I am having problems getting my head around generics in the following situation, see inline comments below for my questions:

public void exampleMethod() {
    //Intuitively I would expect this to mean that test is set containing objects 
    //that subclass AbstractGroup
    Set<? extends AbstractGroup> test;

    //Yet the compiler complains here and I do not understand why?

    test.add(new AnyAbstractGroupSubGroup());

    //I would guess that a method call such as this at runtime

    test = new HashSet<SubGroupA>()

    //would mean that only objects of subgroupA can be added to the collection, but then
    //what is the point in using the wildcard in the first place?  
}

解决方案

//Intuitively I would expect this to mean that test is set containing objects 
//that subclass AbstractGroup
Set<? extends AbstractGroup> test;

Nope, it means that it's a set of one specific ? which extends AbstractGroup. And neither you nor the Compiler have any way of knowing what that ? is, so there's no way you can add anything to that Set.

You can assign the set's values to variables of type AbstractGroup, but not the other way around.

Instead, you need this:

Set<? super AbstractGroup> test;

This principle is sometimes called PECS and explained well in this answer.

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